Electrical contact

ABSTRACT

In an electrical contact arrangement wherein a disconnect assembly having one or more spring loaded electrically conductive contact fingers which slidably engage and disengage from an electrically conductive stud. The fingers and stud are made of copper and the engaging surfaces are provided with a layer of silver and a layer of indium on the silver. The indium layer being on the order of 0.1-1.5 mils thick and diffused along its boundary layer to the silver by heat treatment (160* C. for 2 hours).

United States Patent Nitz [ 51 Feb. 8, 1972 [54] ELECTRICAL CONTACT [72]Inventor: Allen R. Nitz, Wauwatosa, Wis.

[73] Assignee: Allis Chalmers Manufacturing Company,

-Milwaukee, Wis.

[22] Filed: Aug. 15, 1969 [21 Appl. No.: 850,580

52 us. Cl. 200/166C [511 Int Cl. ..ll0lh 1/02 [58] Field of Search..200/l66 C [56] References Cited UNITED STATES PATENTS 2,417,967 3/1947Booe ..200/l66 C x Primary Examiner-H. 0. Jones Attorney-Thomas F.Kirby, Robert B, Benson and Lee H. Kaiser [57] ABSTRACT In an electricalcontact arrangement wherein a disconnect assembly having one or morespring loaded electrically conductive contact fingers which slidablyengage and disengage from an electrically conductive stud. The fingersand stud are made of copper and the engaging surfaces are provided witha layer of silver and a layer of indium on the silver. The indium layerbeing on the order of 0. ll,5 mils thick and diffused along its boundarylayer to the silver by heat treatment (160 C. for 2 hours).

3 Claims, 3 Drawing Figures ELECTRICAL CONTACT SUMMARY OF THE INVENTIONThis invention relates generally to electrical contacts. In particularit relates to silver-plated sliding contacts which have an indiumcoating thereon.

Some types of electrical circuit interrupters are adapted to be slidinto and out of cubicles and are provided at their rear with primarydisconnect assemblies which engage with studs on the bus bars to whichthe breaker is connectable. Each disconnect assembly typically comprisestwo or more spring loaded contact fingers, usually made of silver-platedcopper, arranged in rows and adapted to slidably engage opposite sidesof a stud, usually made of silver-plated copper. Normally, a silverplating on the order of 0.3 mils thick is sufficient to seal the copperfrom tarnishing when no wear is present. However,

repeated withdrawal and reinsertion of the circuit breaker with respectto the cubical causes wear on the contact surfaces of the fingers of thedisconnect assembly and wear on the contact surfaces of the studs.Furthermore, in chemically contaminated atmospheres where concentrationsof hydrogen sulfide or sulfur dioxide are high, tarnishing of thesilvered contact surfaces occurs which ultimately causes electricalfailure of the disconnect assembly due to increased electricalresistance and eventual burnout.

It is known that the metal indium in itspure state or alloyed withsilver or silver alloys has tarnish-resisting characteristics. US. Pat.No. 2,157,933 issued May 9, 1939 to Hensel and Emment and assigned to P.R. Mallory & Co. teaches electrical contacts made of silver-indiumalloy. A paper entitled A New Method for Making Low-Resistance Contactsto Aluminum by R. W. Barnard and J. P. Pasternak presented at the HolmSeminar during Nov. 11 through 15, 1968 teaches, among other things, aconnector of unspecified metal plated with indium for wedging contactwith copper wire or aluminum wire and also teaches crossed aluminum andindiumplated copper wire.

However, none of these teachings offer a satisfactory solution to theproblem encountered by applicant who is concerned with a disconnectcontact having fingers which have a normal contact pressure of about22.4 lbs. per square inch, which are expected to withstand at least 150sets of insertion and withdrawal movements, which are rated at 100amperes per finger continuous duty, and which operate at servicetemperature on the order of 100 C.

For example, indium plating directly on copper components is notsatisfactory because contact pressure will cause the components togradually sink through the indium layer and bear against each otherthereby allowing for exposure of the copper to atmospheric conditionswith consequent increases in contact resistance. Indium-silver alloys asa component material or as a coating on a component offer too high anelectrical resistance for uses such as applicants intended use and alsoraise cost problems. Indium in its pure state is unsatisfactory as acontact material because of its relative softness, highrelativeresistance and its high cost.

In accordance with the presentinvention the fingers of a primarydisconnect assembly and a bus bar stud are made of standard electricalcopper and the cooperating contact surfaces thereof are provided with acoating of silver on the order of 0.3 mils thick on which is depositedby plating or dipping a coating of indium on the order of 0.1-1.5 milsthick, after which the layer of indium adjacent the silver is partiallydiffused into the silver by heating for l to 2 hours at about 160 C.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide improved electrical contacts, particularly, primary disconnecttype contacts.

Another object is to provide improved contacts which have high-currentcarrying capacity, relatively low-electrical resistance, and tend toresist heating.

Another object is to provide improved contacts which can withstandrepeated connection and disconnection with respect to another conductivemember without exhibiting undue mechanical wear and attendant resistanceheating.

Another object is to provide an improved contact which comprises asilver-plated copper body on which a coating of indium is disposed andpartially diffused thereinto along the boundary layer.

Another object is to provide improved disconnect contacts of theaforesaid type which comprise silvered material upon which is disposed acoating of metal selected from the group consisting of indium and goldand wherein the coating is partially diffused into the silver.

Another object is to provide an improved method for making electricalcontacts which comprises the step of applying a coating of indium to asilvered material by electroplating, or chemical plating, or dipping andthe step of heating said coating to effect diffusion of a portion of theindium into the silver.

Other objects and advantages of the invention will hereinafter appear.

DESCRIPTION OF THE DRAWINGS The accompanying drawing illustrates apreferred embodiment of the invention but it is to be understood thatthe embodiment illustrated is susceptible of modification with respectto details thereof without departing from the scope of the appendedclaims.

In the drawing:

FIG. 1 is a top view of a disconnect contact assembly associated with abus bar stud, both in accordance with the present invention;

FIG. 2 is a side elevational view partly in section of the contactassembly and stud of FIG. 1; and

FIG. 3 is an enlarged view, partly in cross section of a contact fingerof the assembly shown in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE DRAWING Referring to FIGS. 1 and 2 of thedrawing, the numeral 10 designates a primary disconnect assembly whichis mounted on an electrically conductive circuit breaker stud 12 andwhich is shown in engagement with an electrically conductive bus bar orcubicle stud 14. It is to be understood, for example, that stud 14 islocated in a cubicle into and out of which an electrical circuit breakercan be slid and that circuit breaker stud 14 is part of such a circuitbreaker. The primary disconnect assembly 10 (hereinafter called adisconnect) serves as an electrical connection between studs 12 and 14and is adapted to disengage from stud 14 when the circuit breaker isslid from the cubicle.

Disconnect 10 comprises a plurality of electrically conductive fingers,each of which is similar to finger 16, which are shown as arranged intwo spaced-apart rows of four fingers each. The fingers in each row arebiased toward each other (and against stud 14) by means of springs suchas spring 18.

It is to be understood that disconnect 10 and cubicle stud 14 are ratedto carry amperes of current per finger during constant duty.Furthermore, they are expected to operate at service temperatures ofabout 100 C. Also, disconnect l0 and cubicle stud 14 are expected towithstand at least sets of insertion or withdrawal movements.

Each finger 16 engages cubicle stud 14 with a normal contact pressure ofabout 22.4 lbs. per square inch under the action of its associatedspring 18.

Referring to FIG. 3, finger 16 is fabricated of a suitable electricallyconductive material 19, such as copper, and is provided with a coating20 of silver on the order of 0.3 mils thick which, preferably, isapplied by electroplating, although chemical plating or dipping could beused. In accordance with the present invention, finger 16 is furtherprovided with a coating 22 of indium on the order of about 0.1-1.5 milthick, which coating can be applied over the silver coating 20 bydipping or by a plating process. After coating.22 is applied,

contact finger 16 is subjected to heating to about 160 C. for from 1 to2 hours to effect diffusion of a small amount of the indium into thesilver at the interface or boundary layer between the two metals.

Referring to FIG. 3, stud 14 which slidably engages with finger 16 isalso fabricated of material 24, such as copper, and is provided with acoating 26 of silver and a coating 28 of indium, except that the silvercoating on stud 14 is typically only 1 mil thick.

When disconnect is moved into engagement with cubicle stud 14, eachcontact finger 16 makes sliding engagement with the cubicle stud, asFIG. 3 shows. The indium coating 22 on finger l6 and the indium coating28 on stud 14 are soft enough to be somewhat displaced because of thespring pressure. Therefore, the undiffused indium serves as anelectrically conductive tarnish resistant lubricant between contactfinger 16 and stud 14. The diffused indium inhibits scratching, marringor scoring on the silver plating on the copper subsurfaces and improvesthe tarnish resistance of the silver plating. Thus, the indiumeffectively protects the silver plating against mechanical damage. Eventhough the indium has a higher coefficient of electrical resistance thaneither silver or copper it does not interfere with the electricalconductivity of the connection between the finger 16 and the stud 14 fortwo reasons. First, the contact pressure is such as to cause the indiumcoating to be squeezed to a dimension thinner than the normal platingthickness of the indium. Second, because of the relative softness of theindium it tends to flow in such a manner as to make contact with agreater area of the surface of finger 16. In this way point contact,line contact, or small surface area contact is avoided and the highercoefficient of resistance of the indium is compensated for by thegreater contact area.

When finger 16 and stud 14 are separated, the lubricating effect of theindium prevents or inhibits contact wear and the indium tends to assumeits normal uniform coating characteristics.

In the embodiment of the invention shown, the fingers l6 and the cubiclestud 14 are shown as having coatings of silver and indium. It is to beunderstood, however, that the finger alone may be provided with thesilver and indium coating and good electrical contact results.Furthermore, although a disconnect assembly is shown, it is apparentthat other types of contacts could be made in accordance with thepresent invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a slidable electrical contact,

an electrically conductive body having a contact region subject totarnishing,

a layer of silver on said contact region of sufficient thickness to sealsaid contact region against tarnishing,

and a layer of indium of a thickness between about 0.1 and about 1.5mils thick on said layer of silver,

said layers of silver and indium being diffused along their boundarylayer.

2. A contact according to claim 7 wherein the contact region has aradius of curvature and is adapted to slidably engage another conductivebody.

3. In combination,

a pair of slidably engageable electrical contacts having tarnishablecontact regions thereon, at least one of said contacts having a contactregion which has a radius of curvature,

a layer of silver on each of said contact regions of sufiicientthickness to seal said surfaces against tarnishing,

and a layer of indium of a thickness between about 0.1 and about 1.5mils thick on each of said layers of silver,

said layers of silver and indium being diffused along their boundarylayers.

2. A contact according to claim 7 wHerein the contact region has aradius of curvature and is adapted to slidably engage another conductivebody.
 3. In combination, a pair of slidably engageable electricalcontacts having tarnishable contact regions thereon, at least one ofsaid contacts having a contact region which has a radius of curvature, alayer of silver on each of said contact regions of sufficient thicknessto seal said surfaces against tarnishing, and a layer of indium of athickness between about 0.1 and about 1.5 mils thick on each of saidlayers of silver, said layers of silver and indium being diffused alongtheir boundary layers.